It can be desirable to produce solid blends of two immiscible thermoplastic polymers, i.e., polymers which do not mix so as to form a homogeneous composition, but which are sufficiently compatible so as to form a generally uniform composition in which each of the polymers exists in distinct domains of the solid composition. To achieve the compatibility of the two polymers that allows them to interface without complete phase separation, it is often necessary to chemically compatibilize the polymers, for example, by grafting compatibilizing chemical moieties to one of the thermoplastic polymers.
The properties of a blended composition of two immiscible polymers is not only a function of the natures of the two polymers and their relative proportions, but also of the phase morphology of the composition, e.g., whether one polymer or the other forms a continuous phase or the sizes of the domains of the immiscible polymers. Applicants have found that the phase morphology may be altered, depending upon how the two immiscible polymers are mixed together and have found that novel phase morphologies can be obtained by varying the normal order of mixing the two polymers. In some cases, it has been found that by using a particular mixing procedure, a particularly desirable blend can be achieved at certain proportions of the two immiscible polymers in which one of the polymers forms a continuous phase with the other polymer existing as distinct, dispersed domains therein.
A particular problem addressed by Applicants was to provide a relatively low-cost gas barrier layer based upon ethylene vinyl alcohol (EVOH). By EVOH is meant, herein, a copolymer of ethylene (between about 20 and about 50 mole percent) and a vinyl ester, e.g., vinyl acetate (between about 50 and about 80 mole percent) which has been substantially saponified (at least about 90 percent) subsequent to polymerization. EVOH exhibits excellent gas barrier properties and is therefore particularly desirable as a gas barrier layer in a multi-layer laminate for forming containers for food. EVOH, however, is a relatively expensive polymer. Manufacturing constraints often demand a layer of a certain thickness, thereby requiring a certain total amount of material in such a layer. It has therefore been proposed that EVOH be blended with less expensive thermoplastic polymers, such as polyolefins; such blends are taught, for example, in U.S. Pat. No. 3,975,463 to Hirata et al., the teachings of which are incorporated herein by reference. U.S. Pat. No. 4,971,864, the teachings of which are incorporated herein by reference, addresses the relatively poor gas barrier provided by EVOH/polyolefin blends, relative to homogeneous EVOH. As the continuous phase of such blends is normally the polyolefin, gas may seep through the polyolefin and around the EVOH domains. The problem is addressed in the U.S. Pat. No. 4,971,864 by extruding in such a manner that in an extruded layer of the blend, there exist overlapping EVOH domains that require that gas travel a tortuous pathway through the layer. However, because the continuous phase is still the polyolefin, a less-than-complete gas barrier is formed.
There exists the need for EVOH/polyolefin blends having better gas barrier properties.
The present invention is directed to producing blends of immiscible polymers with particular phase morphologies. Of interest are blends of two immiscible polymers in which a desired one of the polymers forms a continuous phase with the other polymer existing as dispersed domains therein. Of particular interest are blends of EVOH and a polyolefin, that is modified so as to be able to blend with the EVOH as a two-phase polymer composition in which the EVOH forms a continuous phase containing dispersed discrete domains of the polyolefin.